Lightweight circuit board with conductive constraining cores

ABSTRACT

Prepregs, laminates, printed wiring board structures and processes for constructing materials and printed wiring boards that enable the construction of printed wiring boards with improved thermal properties. In one embodiment, the prepregs include substrates impregnated with electrically and thermally conductive resins. In other embodiments, the prepregs have substrate materials that include carbon. In other embodiments, the prepregs include substrates impregnated with thermally conductive resins. In other embodiments, the printed wiring board structures include electrically and thermally conductive laminates that can act as ground and/or power planes.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.10/020,506, filed Dec. 11, 2001, which claims the benefit of U.S.Provisional Patent Application No. 60/254,997, filed Dec. 12, 2000. Theentire contents of patent application Ser. No. 10/020,506 are expresslyincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Certain portions of the background technology to the present inventionwere made under one or more of the following United States GovernmentContracts: NAS 3-27743; NAS 3-97040; and F 29601-94-C-0093. Accordingly,the United States Government may have some rights.

BACKGROUND OF THE INVENTION

Multiple-layer printed circuit boards or printed wiring boards (PWBs)are used for mounting integrated circuits (ICs) and other components.The push to decrease circuit size and weight and to operate at higherfrequencies and clock speeds has led to smaller components generatinggreater heat and being placed more closely together on the PWB.Additional size and speed improvements have also been achieved byreducing the footprints of the components by using leadless chipcarriers.

The greater density of components on the PWBs and hotter componentsresulted in thermal management problems. The Coefficient of ThermalExpansion (“CTE”) mismatch between the PWBs and the components becomesmore important when greater temperatures are generated. CTE mismatchbetween the PWBs and components can result in fracture or fatigue duringthe thermal cycling caused by powering on and off of electronic devices.Leadless chip carriers are especially susceptible to disengagement fromthe PWB when there are CTE mismatches. Solder joints and connectionstend to pull apart in the “tug-of-war” introduced by the CTE mismatch.

Prior PWB designs have used metal constraining layers or cores, such ascopper-invar-copper, aluminum or steel, to lower the board's CTE.However, these materials add undesirable weight. U.S. Pat. No. 4,318,954to Jensen provides an example of a PWB design for use in cycling thermalenvironments that uses lightweight carbon based constraining layers tolower the board's CTE. U.S. Pat. No. 4,591,659 to Leibowitz alsodemonstrates that carbon constraining layers can serve as thermalconductors for carrying heat away from the components mounted on the PWBin addition to lowering the board's CTE. U.S. Pat. No. 4,318,954 toJensen and U.S. Pat. No. 4,591,659 to Leibowitz are incorporated byreference in their entirety to the present disclosure.

The ability of previous PWBs to conduct heat away from the componentsmounted on their surfaces is limited by the prepreg used to preventelectrical conductivity between the functional layers of the PWB. Thematerials used in prepreg have poor thermal conduction properties.Therefore, the ability of the carbon constraining layer to conduct heataway from the surface of the board was limited by the amount of prepregbetween it and the surface of the board. The carbon material used in thecarbon constraining layers is electrically conductive, which requiredthe functional layers of the PWB in prior structures to be electricallyinsulated from the carbon constraining layers in order to prevent shortcircuits and cross talk. In previous designs, this requirement places alower limit on the distance between the carbon constraining layers andthe surface of the board equivalent to the minimum amount of prepregrequired to insulate the functional layers of the board from each otherand from the carbon constraining layers. This lower limit translatedinto an upper limit on the amount of heat that could be conducted awayfrom the surface of the PWB. Accordingly there was a need for a PWB thatpossessed mechanical strength with a low CTE and that exceeds the upperlimit on the amount of heat that can be conducted away from the surfaceof the PWB which was inherent in previous designs.

SUMMARY OF THE INVENTION

In one aspect, the invention relates to a structure and method in whicha thermally conductive layer is provided in a PWB or a portion thereof.For example, the invention may include a prepreg layer made of asubstitute impregnated with a resin which is thermally conductive, andpossibly also electrically conductive. A laminate may be formed fromsuch a prepreg layer, the laminate having first and second metalliclayers positioned above and below the prepreg. Alternatively, thelaminate may itself be thermally and/or electrically conductive,enabling its use in a high performance printed wiring board.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a semi-schematic cross-sectional view showing a PWB inaccordance with the present invention including an electrically andthermally conductive laminate;

FIG. 2A is a flow chart illustrating a process for constructing a PWB inaccordance with the present invention;

FIG. 2B is a flow chart illustrating a process for impregnating asubstrate with resin in accordance with the present invention;

FIG. 3 is a semi-schematic cross-sectional view showing a laminateincorporating four layers of unidirectional carbon fibers;

FIG. 4 is a semi-schematic cross-sectional view showing anotherembodiment of a laminate incorporating four layers of unidirectionalcarbon fibers;

FIG. 5 is a semi-schematic cross-sectional view showing a laminateincorporating three layers of unidirectional carbon fibers;

FIG. 6 is a semi-schematic cross-sectional view showing a laminateincorporating four layers of unidirectional carbon fibers in anisotropic configuration;

FIG. 7 is a semi-schematic cross-sectional view showing a PWB inaccordance with the present invention including a laminate incorporatingprepreg layers;

FIG. 8 is a semi-schematic cross-sectional view showing a PWB inaccordance with the present invention including an electrically andthermally conductive laminate including a fiber glass layer impregnatedwith electrically and thermally conductive resin;

FIG. 9 is a semi-schematic cross-sectional view showing a PWB inaccordance with the present invention including an electrically andthermally conductive laminate having a fiber glass layer impregnatedwith electrically and thermally conductive resin contained within layersof prepreg;

FIG. 10 is a semi-schematic cross-sectional view showing a PWB inaccordance with the present invention including two electrically andthermally conductive laminates and a number of chimneys and platedthrough holes;

FIG. 11A is a flow chart illustrating a process for manufacturing a PWBin accordance with the present invention including multiple electricallyand thermally conductive laminates, chimney holes and plated throughholes;

FIG. 11B is a flow chart illustrating a process for determininglocations in which to drill chimney holes in a PWB;

FIG. 11C is a flow chart illustrating a process for determininglocations in which to drill filled clearance holes in electrically andthermally conductive laminates during the construction of a PWB inaccordance with the present invention; and

FIG. 12 is a semi-schematic cross-sectional view showing a PWB inaccordance with the present invention including two electrically andthermally conductive laminates and an electrically isolated carbonsupport layer.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, a printed wiring board (“PWB”) 10″″ inaccordance with an embodiment of the present invention is shown in FIG.10. The PWB includes a first laminate 120, and a second laminate 122,multiple layers of prepreg 124 and multiple layers of metal 126. The PWB10″″ contains circuits and is used for mounting integrated circuits(ICs) and components. The term circuit is used to describe anelectrically conductive path between two or more points. Individuallayers of the PWB can include circuits and a number of circuits onseveral layers of the PWB can be connected to create an overall PWBcircuit. The layers on which circuits are located are often referred toas functional layers.

The laminates 120 and 122 comprise a carbon containing layer 14sandwiched between a first layer of metal or other electricallyconductive material 16 and a second layer of metal or other electricallyconductive material 18. Both of the laminates 120 and 122 areelectrically conductive, which enables the laminate to be used as afunctional layer within the PWB. The functions that can be performed bythe laminates include acting as a ground plane within the PWB, a powerplane within a PWB or both a ground and power plane in the PWB whererouting is used to electrically isolate portions of the laminate.Various examples of other laminate structures that can be used inaccordance with the present invention to implement the laminates 120 and122 are discussed below.

The layers of metal can act as functional layers in the PWB. In oneembodiment, the layers of metal or other electrically conductivematerial are patterned with electrical circuits. Electrical contactbetween the various layers of metal or laminates can result in thefunctions of the electrical circuits patterned onto the layers of metalbeing interrupted. Therefore, prepregs are used to electrically insulatethe electrically conductive laminates 120 and 122 and the layers ofmetal 126.

A prepreg is a composite layer that includes a substrate or supportingmaterial composed of fibrous material that is impregnated with resin.The prepregs are electrical insulators having dielectric constants lessthan 6.0 at 1 MHz. A prepreg may also be a film. A film is a type ofprepreg that does not include a substrate but is instead a compositethat only includes resins. Materials that can be used to constructprepregs in accordance with the present invention are discussed below.

Often, the circuits within a PWB include plated “through holes” toestablish connections between the functional layers of the PWB. In oneembodiment, the PWB 10″″ includes through holes 130 lined withelectrically conductive material that are used to establish electricalconnections between the functional layers in the PWB. These linedthrough holes enable electrical signals to pass between circuits on themetal layers and/or the laminates. It is well known in the art thatconnections can be created between the electrically conductive liningsof plated through holes and circuits patterned on a layer of metal bylocating the plated through hole such that the lining of the platedthrough hole contacts a portion of the circuit patterned on the layer ofmetal. When a connection between a laminate and a plated through hole isdesired, the through hole is simply drilled through the laminate at thedesired location and an electrical connection is established where theelectrically conductive lining of the through hole contacts theelectrically conductive laminate.

Techniques for avoiding electrical connections between circuitspatterned on a layer of metal in a PWB and a plated through hole arewell known in the art. Each of the options essentially involvesdesigning the circuit routings and the locations of the plated throughholes to avoid contact between the electrically conductive lining of theplated through hole and the circuit. Where connections between theplated through holes and the first or second laminates are not desired,then an annulus of dielectric material 132 such as an epoxy resin with adielectric constant less than 6.0 at 1 MHz can be used to ensure that anelectrical connection does not exist between the laminate and theelectrically conductive lining of the through hole.

A process in accordance with the present invention for manufacturing thePWB 10″″ illustrated in FIG. 10 is shown in FIG. 11A. The process 150commences with the step 152, which involves constructing two laminatesin accordance with the present invention using any of the appropriateprocesses described below including the process illustrated in FIG. 2A.Power or ground regions are then patterned on the laminates in the step154.

Oce the patterning is complete, the laminates are subjected to oxidetreatment in the step 156. After oxide treatment, clearance holedrilling is performed in the step 158. Clearance hole drilling involvesdrilling holes in the laminate of a first diameter and filling theresulting holes with a dielectric material such as any of the resinsdescribed below with a dielectric constant less than 6.0 at 1 MHz. Priorto filling the drilled holes, they are inspected and cleaned using highpressure dry air.

Once the clearance holes have been drilled, the second lamination cycleis performed in the step 160. The second lamination cycle is similar tothe second lamination cycle described below in relation to FIG. 2A.After the second lamination cycle, circuits are etched onto the layersof metal that will be located within the interior of the finished PWBare patterned in the step 166 and then subjected to oxide treatment inthe step 168.

Following the oxide treatment, the third lamination step is performed inthe step 170. The third lamination involves aligning the two structuresproduced in the second lamination with additional prepreg layers tocorrespond with the layers of the PWB 10″″ illustrated in FIG. 10. Thelayers are then exposed to temperatures and pressures similar to thoseexperienced during the second lamination cycle.

After the third lamination cycle, the final through hole drilling isperformed in step 172. The final through hole drilling involves drillingholes through the entire PWB that have a second diameter, which is lessthan the first diameter described above. The through holes are thenlined in the step 174. Preferably, the through holes are lined withcopper. In other embodiments, the through holes can be plated withmaterials similar to those that can be used in the construction of thelayers of metal. If a through hole passes through one of the filledclearance holes in a laminate, then the lining of the through holes areelectrically isolated from the laminate in which the clearance hole isdrilled. If a through hole does not pass through one of the filledclearance holes in a laminate, then the lining of the through holes isin electrical contact with the laminate.

A lightweight multiple-layer PWB in accordance with the presentinvention is illustrated in FIG. 1. The PWB 10 includes a laminate 12comprising a carbon containing layer 14 sandwiched between a first layerof metal or other electrically conductive material 16 and a second layerof metal or other electrically conductive material 18. The laminate issandwiched between a first layer of prepreg 20 and a second layer ofprepreg 22. The top layer of the PWB is constructed from a third layerof metal or other electrically conductive material 24. The bottom layerof the PWB is constructed using a fourth layer of metal or otherelectrically conductive material 26. As set forth below, theelectrically conductive layers 16, 18, 24 and 26, and the correspondinglayers of the other embodiments described herein, may be made of metalor any of a variety of metal-containing compositions having suitableproperties of electrical conduction. For convenience, however, theselayers often will be referred to herein simply as “metal” layers.

The laminate 12 is electrically conductive, which enables the laminateto be used as a ground plane within the PWB, a power plane within a PWBor both a ground and power plane in the PWB where routing is used toelectrically isolate portions of the laminate. Use of the laminate 12 ina PWB results in the PWB being thinner and having less weight thanprevious PWB designs that utilize electrically insulated carboncontaining layers to lower the CTE. Reducing the thickness of the PWB 10also enables the carbon containing layers 14 to be located closer to thesurface of the board than in PWBs that utilize electrically insulatedcarbon constraining layers. An advantage of this configuration is thatit gives the PWB increased ability to transfer heat away from itssurface compared to previous designs. Another advantage of thisconfiguration is that it provides low surface CTE which is important inapplications such as semiconductor applications.

A prepreg is a composite layer that includes a substrate or supportingmaterial composed of fibrous material that is impregnated with resin. Aprepreg may also be a film. A film is a type of prepreg that does notinclude a substrate but is instead a composite that only includesresins. The first prepreg layer 20 and the second prepreg layer 22electrically insulate the electrically conductive laminate 12 from thethird layer of metal 24 and the fourth layer of metal 26. In onepreferred embodiment, the third and fourth layers of metal or otherelectrically conductive material are patterned with electrical circuits.For example, electrical contact between the third layer of metal, theelectrically conductive laminate or the fourth layer of metal can resultin the functions of the electrical circuits patterned onto the third andfourth layers of metal being interrupted. In other embodiments, only oneof the third and fourth layers of metal are patterned with electricalcircuits.

In one preferred embodiment of the PWB in accordance with the presentinvention, the layer containing carbon used in the construction of thelaminate 12 is made from woven carbon fibers such as woven K13C2Umanufactured by Mitsubishi Chemical America, Inc. of Sunnyvale, Calif.and having a thickness of 0.006 inches. In another embodiment, the layercontaining carbon can be constructed from carbon fibers having a tensilemodulus of 110 msi, a tensile strength of 540 ksi, a thermalconductivity of 610 W/m.K, a fiber density of 2.15 g/cc and a fiberelongation of 0.5% and that are woven with a balanced weave. In otherembodiments, the layer containing carbon can be constructed from anywoven carbon fibers having a thickness greater than 0.002 inches, athermal conductivity greater than 10 W/m.K, a co-efficient of thermalexpansion in the range −3.0 to 3.0 ppm/C, a stiffness greater than 20msi, a tensile greater than 250 ksi, a density less than 2.25 gm/cc.Preferably, the layer containing carbon is constructed from woven carbonfibers having a thermal conductivity greater than 75 W/m.K, aco-efficient of thermal expansion in the range −1.25 to 1.0 ppm/C, astiffness greater than 35 msi, a tensile strength greater than 350 ksi,a density less than 2.22 gm/cc. More preferably, the layer containingcarbon is constructed from woven carbon fibers having a co-efficient ofthermal expansion of 0.0 ppm/C. In other embodiments, the layercontaining carbon is constructed from any woven carbon fibers capable ofdissipating the required amount of heat from the surface of the PWB 10,to support the CTE requirements of the PWB and to achieve the desiredstiffness of the PWB.

In one preferred embodiment, the woven carbon fibers are impregnatedwith an electrically and thermally conductive resin such as an epoxypyrolitic carbon resin in accordance with the process described above inrelation to FIG. 2B. Electrical conductivity is defined as having adielectric constant greater than 6.0 at 1 MHz. Thermal conductivity isdefined as having a thermal conductivity of greater than 1.25 W/m.K.Preferably, a material that is thermally conductive will have a thermalconductivity greater than 2.5 W/m.K. In other embodiments, the wovencarbon fibers are impregnated with a resin such as polyimide (cyanateester) based pyrolitic carbon resin, epoxy or polyimide based silveroxide resin, epoxy or polyimide based carbon powder resin or any otherresin having a glass transition temperature greater than 100° F., lowmoisture absorption, high resistance to chemical corrosion, highresistance to microcracking, high structural durability, controlledflow, good adhesion, a thermal conductivity greater than 0.2 W/m.k and adielectric constant greater than 6.0 at 1 Mhz. Preferably, the wovencarbon fibers are impregnated with a resin having a glass transitiontemperature greater than 250° F. and a thermal conductivity greater than2.0 W/m.k.

In one preferred embodiment, the first and second layers of metal areconstructed from a ¼ oz copper foil such as NT-TW-HTE manufactured byCircuit Foil Trading, Inc of Glenside, Philadelphia. In otherembodiments, other electrically conductive materials such as Cu,Palladium, Ag, Al, Au, Ni and Sn, or alloys or other compositionsthereof, having thicknesses from 0.00003 inches to 0.021 inches can beused in the construction of the first and second layers of metal. Inother embodiments, an electrically conductive material of any thicknesscan be used in the construction of the first and second layers of metalprovided that the overall conductivity of the electrically conductivelaminate 12 is sufficient to carry the electrical load in the laminate.

In one preferred embodiment, the first prepreg layer and the secondprepreg layer are constructed from thermally conductive dielectricmaterial such as the prepreg 44N0680 manufactured by Arlon Materials forElectronics of Rancho Cucamonga, California having a thickness of 0.0015inches, a resin content of approximately 80%, a resin flow ofapproximately 50% and a gel time in the range of 90 to 110 seconds. Inother embodiments, other prepregs such as FR-4, polyimide, teflon,ceramics, GIL, Gtek or high frequency circuit materials manufactured byRogers Corporation that include additives such as aluminum oxide,diamond particles or boron nitride or any other prepreg havingdielectric constants less than 6.0 at 1 MHz and a thermal conductivityof greater than 1.25 W/m.K can be used in the construction of the firstand second prepreg layers. More preferably, the first and second prepreglayers are constructed from a dielectric material having a dielectricconstant less 4.0 at 1 Mhz and a thermal conductivity greater than 2.0W/m.K. In other embodiments, prepregs that have thermal conductivityless than 1.25 W/m.K can be used in the construction of the first andsecond prepreg layers. Use of prepreg layers that have a thermalconductivity that is less than 1.25 W/m.K can reduce the ability of thePWB to conduct heat away from its surface.

In one preferred embodiment the top and bottom layers of conductivematerial are constructed from materials similar to those used in theconstruction of the first and second layers of metal as described above.

One preferred embodiment of a method of manufacturing PWBs in accordancewith the present invention is illustrated in FIG. 2A. A first laminationis performed in the step 32. The first lamination involves placing a ¼oz layer of copper foil on one side of a layer of woven carbon fibersimpregnated with the epoxy based pyrolitic carbon resin, as describedabove, and placing a second ¼ oz layer of copper foil on the other sideof the layer of woven carbon fibers. The layers are then placed in avacuum and heated from room temperature to 350° F. The temperatureincrease is controlled so that the temperature rise is maintained withinthe range of 8-12° F./min as the temperature rises from 150° F. to 300°F. When the temperature is in the range 150° F.-165° F., the pressure onthe layers is increased to 250 PSI. Once a temperature of 350° F. hasbeen reached, the temperature is maintained at that temperature for 70minutes. After the completion of the 70 minute time period, the layersare exposed to room temperature and a pressure greater than atmosphericpressure for a period of 30 minutes. The first lamination cycle producesthe electrically conductive laminate 12 described above. Preferably, theelectrically conductive laminate is manufactured to be as flat aspossible.

The first lamination cycle is followed by a second lamination cycle inthe step 34. The second lamination cycle involves placing a layer44N0680 prepreg on one side of the electrically conductive laminateproduced in the first lamination cycle and a second layer of 44N0680prepreg on the other side of the electrically conductive laminate. Inaddition, layers of ½ oz copper foil are placed on the outside surfacesof the two layers of 44N0680 prepreg. The layers are then placed in avacuum and heated from room temperature to 350° F. The temperatureincrease is controlled so that the temperature rise is maintained withinthe range of 8-12° F./min as the temperature rises from 150° F. to 300°F. When the temperature is in the range 150° F.-165° F., the pressure onthe layers is increased to 250 PSI. Once a temperature of 350° F. hasbeen reached, the temperature is maintained at that temperature for 90minutes. After the completion of the 90 minute time period, the layersare exposed to room temperature and a pressure greater than atmosphericpressure for a period of 30 minutes. The first lamination cycle producesthe electrically conductive laminate 12 described above. The secondlamination cycle produces the PWB 10 shown in FIG. 1. The third andfourth layers of metal of the PWB 10 shown in FIG. 1 are then patternedwith electrical circuits in the step 36.

One preferred embodiment of a process 40 for impregnating a layercontaining carbon constructed from woven carbon fiber with anelectrically conductive resin is illustrated in FIG. 2B. The first stepin the process 42 involves adding together ingredients to form a resin.Most resins are formed using epoxy or polyimide solid state resin,solvent, acetones, catalysts and additives. Typically, the properties ofa particular resin are determined by the various additives included inthe resin and the quantities of these additives. Additives can be usedto increase the electrical conductivity or the thermal properties of aresin. When an additive is used to increase the electrical or thermalconductivity of a resin, the thermal or electrical conductivity of aresin increases with the amount of the additive mixed through the resin.In one preferred embodiment, an amount of pyrolitic carbon in powderform equal to 10% by weight of the resin is added as an ingredient toincrease the electrical conductivity and thermal properties of theresin. In other embodiments, any amount of pyrolitic carbon can be addedto improve the thermal and electrical properties of the resin.Preferably, the amount of pyrolitic carbon added to the resin is between5% to 50% by weight of the resin. The various resin ingredients are thenmixed in the step 44 to form a substantially homogenous resin.

Once a resin is formed, the resin is placed in a prepreg treater in thestep 46. The prepreg treater is used to impregnate a substrate withresin. In the next step 48, the substrate to be impregnated is passedthrough the prepreg treater. In one preferred embodiment of the process,the substrate material is woven carbon fibers such as the woven carbonfiber materials described above. In one preferred embodiment using awoven carbon fiber substrate, the substrate is impregnated with 45% byweight resin. In other embodiments, the substrate is impregnated withbetween 5% to 80% by weight resin.

Once the substrate has been passed through the prepreg treater, theB-stage curing cycle is performed in the step 50. The B-stage curingcycle involves exposing the substrate and resin to a temperature ofbetween 250° F. to 300° F. The amount of time that the substrate andresin are exposed to this temperature is determined by the amount ofresin loaded onto the substrate and the extent of curing required. Inone preferred embodiment, a time period of 15 minutes is required forthe impregnation of a woven carbon fiber substrate with 45% resin curedto B-stage so that it is suitable for use in the process described abovein relation to FIG. 2A. Upon the completion of the B-stage curing cycle,the resin is stored in a controlled environment prior to use in the step52.

In other embodiments, silver oxide particles are used as a resinadditive to increase the electrical conductivity and thermal propertiesof the resin. In one preferred embodiment, an amount of silver oxideequal to 40% by weight of the resin is added. In other embodiments, anyamount of silver oxide can be added to increase the thermal propertiesof the resin. Preferably, the amount of silver oxide added to the resinwill be between 5% and 70% by weight of the resin.

In other embodiments, boron nitride particles are used as a resinadditive to increase the thermal properties of the resin. In onepreferred embodiment, an amount of boron nitride equal to 40% by weightof the resin is added. In other embodiments, any amount of boron nitridecan be added to increase the thermal properties of the resin.Preferably, the amount of boron nitride added to the resin will bebetween 5% and 70% by weight of the resin.

In other embodiments, diamond particles are used as a resin additive toincrease the thermal properties of the resin. In one preferredembodiment, an amount of diamond particles equal to 15% by weight of theresin is added. In other embodiments, any amount of diamond particlescan be added to increase the thermal properties of the resin.Preferably, the amount of diamond particles added to the resin will bebetween 2% to 50% by weight of the resin.

In other embodiments, aluminum oxide particles are used as a resinadditive to increase the thermal properties of the resin. In onepreferred embodiment, an amount of aluminum oxide equal to 40% by weightof the resin is added. In other embodiments, any amount of aluminumoxide can be added to increase the thermal properties of the resin.Preferably, the amount of aluminum oxide added to the resin will bebetween 5% to 70% by weight of the resin. In other embodiments, two ormore of the additives described above can be used as additives to form aresin.

In other embodiments, prepregs can be manufactured using the aboveprocess by using substrate materials that have dielectric constants lessthan 6.0 at 1 MHz. In one preferred embodiment, a fiberglass substrateis impregnated with a resin containing boron nitride to produce athermally conductive prepreg with a dielectric constant less than 6.0 at1 MHz. Preferably, the fiberglass is impregnated with 70% by weightresin. In other embodiments, the fiberglass is impregnated with between20% and 80% by weight resin.

In other embodiments, other substrates such as kevlar, quart, aramid orany other material or mixture of materials having a dielectric constantless than 6.0 at 1 MHz, a glass transition temperature greater than 250°F., a thermal conductivity greater than 0.1 W/m.K, a CTE between −4.5ppm/° C. and 30 ppm/° C., high tensile strength and high thermalendurance can be used in the construction of prepreg layers. Preferably,the substrate material has a glass transition temperature greater than400° F., a CTE between −4.5 ppm/° C. and 12 ppm/° C., retains 50% to 60%of its strength at 700° F. and has a dielectric constant less than 3.0at 1 Mhz. Prepreg manufactured using this process can be used in theconstruction of the first and second prepreg layers of the PWB 10 inaccordance with the present invention illustrated in FIG. 1.

In other embodiments, the layer containing carbon is impregnated with aresin that is thermally conductive such as a epoxy or polyimide basedboron nitride resin, epoxy or polyimide based aluminum oxide, epoxy orpolyimide based ceramic resin, epoxy or polyimide based diamondparticles resin or any other resin having properties similar to theelectrically and thermally conductive resins described above except thatthe dielectric constant of the resin is less than 6.0 at 1 Mhz.

In other embodiments, the layer containing carbon is constructed from asheet of unidirectional carbon fiber such as unidirectional K13C2Umanufactured by Mitsubishi Chemical America, Inc. and having a thicknessof 0.001 inches. The unidirectional carbon fiber material chosen for usein the construction of the carbon containing layer preferably hasproperties similar to those described above for the woven carbon fiberthat can be used in the construction of the layer containing carbon.

In other embodiments, the sheet of unidirectional carbon fiber isimpregnated with resin. Resins with similar properties to thosedescribed above in relation to embodiments of laminates incorporatingsheets of woven carbon fiber can also be used to impregnate sheets ofunidirectional carbon fiber used in the construction of laminates inaccordance with the present invention.

In other embodiments, multiple layers of unidirectional carbon fiberthat are aligned such that the fibers in each of the layers aresubstantially parallel can be used in the construction of the layercontaining carbon.

One preferred embodiment of a laminate 12′ constructed in accordancewith the present invention using four unidirectional layers of carbonfiber is illustrated in FIG. 3. In this embodiment, the laminate isconstructed from a first unidirectional layer of carbon fiber 60, asecond unidirectional layer of carbon fiber 62, a third unidirectionallayer of carbon fiber 64 and a fourth unidirectional layer of carbonfiber 66. Each of the unidirectional layers of carbon fiber have thesame thickness and fiber area weight. The first and fourthunidirectional layers of carbon fiber are constructed so that the carbonfibers in each layer are aligned to be substantially parallel. Thesecond and third unidirectional layers of carbon fiber are constructedfrom sheets of unidirectional carbon fiber, where the fibers are alignedsubstantially perpendicular to the carbon fibers in the first and fourthlayers.

Another preferred embodiment of a laminate 12″ constructed in accordancewith the present invention using unidirectional layers of carbon fiberare illustrated in FIG. 4. In this embodiment the laminate 12″ isconstructed from a first unidirectional layer of carbon fiber 70, asecond unidirectional layer of carbon fiber 72, a third unidirectionallayer of carbon fiber 74 and a fourth unidirectional layer of carbonfiber 76. Each of the unidirectional layers of carbon fiber have thesame thickness and fiber area weight. The first and third unidirectionallayers of carbon fiber are constructed from sheets of unidirectionalcarbon fiber having fibers aligned in substantially the same direction.The second and fourth unidirectional layers of carbon fiber areconstructed from sheets of unidirectional carbon having fibers alignedin a direction substantially perpendicular to the direction in which thefibers in the first and third unidirectional layers of carbon fiber arealigned.

Another preferred embodiment of a laminate 12′″ constructed inaccordance with the present invention using unidirectional layers ofcarbon fiber are illustrated in FIG. 5. In this embodiment the laminate12′″ is constructed from a first unidirectional layer of carbon fiber 80having a thickness of 0.002 inches, a second unidirectional layer ofcarbon fiber 82 having a thickness of 0.004 inches and a thirdunidirectional layer of carbon fiber 84 having a thickness of 0.002inches. The fiber area weight of the first and third unidirectionallayers of carbon fiber have the same fiber area weight, which is halfthe fiber area weight of the second unidirectional layer of carbonfiber. The first and third unidirectional layers of carbon fiber areconstructed from sheets of unidirectional carbon fiber having fibersaligned in the same direction. The second unidirectional layer of carbonfiber is constructed from a sheet of unidirectional carbon having fibersaligned in a direction perpendicular to the direction in which thefibers in the first and third unidirectional layers of carbon fiber arealigned.

In other embodiments, a number of layers of unidirectional carbon fibergreater than four can be used in the construction of the printed circuitboard provided that the layer containing carbon fiber is balanced.

In other embodiments, laminates in accordance with the present inventioninclude layers containing carbon that are substantially isotropic. Oneembodiment of a laminate in accordance with the present inventionincorporating an isotropic carbon containing layer is illustrated inFIG. 6. The laminate 12″″ includes a first unidirectional layer ofcarbon fiber 90 constructed from a sheet of unidirectional carbon fiberwith fibers aligned in a first reference direction, a secondunidirectional layer of carbon fiber 92 constructed from a sheet ofunidirectional carbon fiber positioned so that its fibers are aligned atan angle of 45° to the first reference direction, a third unidirectionallayer of carbon fiber 94 constructed from a sheet of unidirectionalcarbon fiber positioned so that its fibers are aligned at an angle of90° to the first reference direction and a fourth unidirectional layerof carbon fiber 96 constructed from a sheet of unidirectional carbonfiber positioned so that its fibers are aligned at an angle of 135° tothe first reference direction. The sheets of unidirectional carbon fibercan be impregnated with resins similar to those resins described above.

A PWB in accordance with the present invention is illustrated in FIG. 7.The PWB 10′ includes a laminate structure 12′″″ having a carboncontaining layer 14′ positioned between a first layer of prepreg 100 anda second layer of prepreg 102. A first layer of metal 16′ is positionedabove the first prepreg layer and a second layer of metal 18′ ispositioned beneath the second prepreg layer. A third layer of prepreg20′ is positioned above the first layer of metal and a second layer ofprepreg 22′ is positioned below the second layer of metal. A third layerof metal 24′ is positioned above the third layer of prepreg and a fourthlayer of metal 26′ is positioned below a fourth layer of prepreg.

In one preferred embodiment, the layer containing carbon 14′ isconstructed from a woven sheet of carbon fiber and the layers of metalare constructed from materials similar to those described above in theconstruction of the layers of metal used in the construction of theembodiment of the PWB shown as 10 in FIG. 1. In addition, third andfourth prepreg layers are constructed from materials similar to thosedescribed above in the construction of the first and second prepreglayers of the embodiment of the PWB shown as 10 in FIG. 1.

In one preferred embodiment an electrically and thermally conductiveprepreg layer such as epoxy based pyrolitic carbon resin prepregmanufactured in accordance with the process described above in relationto FIG. 2B, and having properties similar to the pyrolitic carbon resindescribed above, is used in the construction of the first and secondprepreg layers. An electrically and thermally conductive prepreg is usedin the construction of the first and second prepreg layers to ensurethat an electrically conductive path exists between the layer containingcarbon and the first and second electrically conductive layers. In otherembodiments, other electrically and thermally conductive prepregs suchas polyimide based pyrolitic carbon resin prepreg, epoxy or polyimidebased silver oxide resin prepreg or any other prepreg having a glasstransition temperature greater than 100° F., low moisture absorption,high resistance to chemical corrosion, high resistance to microcracking, high structural durability, controlled flow, good adhesion, athermal conductivity greater than 0.2 W/m.k and a dielectric constantgreater than 6.0 at 1 Mhz can be used in the construction of the firstand second prepreg layers. Preferably, the first and second prepreglayers are constructed from a prepreg having a glass transitiontemperature greater than 250° F. and a thermal conductivity greater than2.0 W/m.k.

The method of manufacturing the PWB 10′ illustrated in FIG. 7 is similarto the method illustrated in FIG. 2A. A layer of epoxy based pyroliticcarbon resin prepreg is placed on one side of a layer of woven carbonfibers and a second layer of epoxy based pyrolitic carbon resin prepregis placed on the other side of the layer of woven carbon fiber. Layersof ¼ oz copper foil are then placed on the outside surfaces of thelayers of epoxy based pyrolitic carbon resin prepreg. These layers arethen subjected to the first lamination cycle as described above inrelation to FIG. 2A to produce the laminate 12′″″. The second laminationcycle and the patterning of the PWB 10′ are also similar to processesdescribed above in relation to FIG. 2A.

In other embodiments, thermally conductive prepreg layers similar tothose used in the construction of the first and second prepreg layers ofthe embodiment of the PWB 10 shown in FIG. 1, as described above, can beused in the construction of the first and second prepreg layers of theembodiment of the PWB 10′ shown in FIG. 7. In embodiments of the PWB 10′that use thermally conductive prepreg layers that are poor conductors ofelectricity, electrical contacts are made between the first and secondlayers of metal and the carbon containing layer by plated through holes.Plated through holes are holes drilled through the laminate 12′″″ thatare lined with electrically conductive material and establish electricalcontacts between the first and second layers of metal and the layercontaining carbon.

In other embodiments, the laminate 12′″″ is constructed from a layercontaining carbon made from layers of unidirectional carbon fibers thathave arrangements similar to the arrangements described above inrelation to the embodiments of laminates in accordance with the presentinvention illustrated in FIGS. 3-6. In other embodiments, the layers ofunidirectional carbon fibers are impregnated with resins similar tothose described above prior to the construction of the laminate 12′″″.

In other embodiments, the laminate 12′″″ is constructed from a layercontaining carbon that is made from a carbon composite sheet or platesuch as a carbon plate manufactured by Mitsubishi Chemical America, Inchaving a thickness of 0.001 inches. A carbon composite sheet or platecan be constructed using a compressed powder mold. In other embodiments,the layer containing carbon can be constructed from any carbon compositesheet or plate has physical properties similar to those described abovein relation to woven carbon fiber.

For embodiments of the laminate 12′″″ constructed using layerscontaining carbon made from carbon composite sheets or plates, the firstlayer of prepreg 100 and the second layer of prepreg 102 can beconstructed from resins similar to those described above.

A PCB in accordance with the present invention is illustrated in FIG. 8.The PCB 10″ includes an electrically and thermally conductive layer 110.A first layer of metal 16″ is positioned above the electrically andthermally conductive layer and a second layer of metal 18″ is positionedbelow the electrically and thermally conductive layer. A first prepreglayer 20″ is positioned above the first layer of metal and a secondprepreg layer 22″ is positioned below the second layer of metal. A thirdlayer of metal 24″ is positioned above the first prepreg layer and afourth layer of metal 26″ is positioned below the second prepreg layer.The electrically and thermally conductive layer and the first and secondlayers of metal form an electrically conductive laminate 112.

In one preferred embodiment, similar materials to those that can be usedin the construction of the embodiment of the PWB 10 in accordance withthe present invention illustrated in FIG. 1 can also be used in theconstruction of the first and second prepreg layers and the first,second, third and fourth layers of metal. In one preferred embodiment,the third and fourth layers of metal are patterned to contain electricalcircuits.

In one preferred embodiment, the electrically and thermally conductivelayer is constructed from a woven fiberglass substrate impregnated withan electrically conductive epoxy pyrolitic carbon resin in accordancewith the process described above in relation to FIG. 2B. Preferably, thewoven fiberglass used in the construction of the electrically andthermally conductive layer is E-glass manufactured by JPS Glass locatedat South Cickering of Ontario in Canada. In other embodiments, othersubstrate materials such as non-woven fiberglass, kevlar, quartz, aramidor other materials having a glass transition temperature greater than250° F., a thermal conductivity greater than 0.1 W/m.K, a CTE between−4.5 ppm/° C. and 30 ppm/° C., high tensile strength and high thermalendurance. Preferably, the substrate has a glass transition temperaturegreater than 400° F., a CTE between −4.5 ppm/° C. and 12 ppm/° C.,retains 50% to 60% of its strength at 700° F. Preferably, the fiberglasssubstrate is impregnated with 70% by weight resin of an epoxy resincontaining 10% by weight pyrolitic carbon additive. In otherembodiments, the electrically and thermally conductive layer is formedusing a substrate that is impregnated with between 5% to 80% of any ofthe resins described above having a dielectric constant greater than 6.0at 1 MHz. In other embodiments, any resin and substrate combination canbe used that results in the electrically and thermally conductive layer14′ having a dielectric constant greater than 6.0 at 1 Mhz.

The embodiment of the PWB 10″ illustrated in FIG. 8 can be manufacturedin accordance with the processes illustrated in FIG. 2A and FIG. 2B.

An embodiment of a PWB in accordance with the present invention isillustrated in FIG. 9. The PWB 10′″ is similar to the PWB 10′illustrated in FIG. 7 except that the carbon containing layer isreplaced with any of the substrate materials described above and thatthe first and second prepreg layers possess dielectric constants greaterthan 6.0 at 1 MHz.

The embodiments of PWBs described above have utilized a single laminate.In other embodiments of PWBs in accordance with the present invention,multiple laminates can be used.

A PWB in accordance with the present invention including two laminatesis illustrated in FIG. 10. The PWB 10″″ includes a first laminate 120, asecond laminate 122, multiple layers of prepreg 124 and multiple layersof metal 126. In one preferred embodiment, the first laminate forms aground plane and the second laminate forms a power plane. In otherembodiments, the function of the laminates can be reversed, bothlaminates can share the same functions or the laminates can be utilizedfor their improved thermal properties only. The use of multiplelaminates can increase the ability of the PWB to conduct heat away fromits surface, improve the CTE of the PWB and can decrease the thicknessand weight of the PWB, when compared to prior art PWBs.

In one preferred embodiment, the first laminate 120 and second laminate122 are constructed similarly to the laminate 12 of FIG. 1. In otherembodiments any of the laminate structures described above can be usedin the construction of the first or second laminate. Preferably, thelayers of prepreg 124 and layers of metal are constructed from materialssimilar to those that can be used to construct the prepreg layers andthe layers of metal in the PWB 10 illustrated in FIG. 1. In otherembodiments any of the laminates described above can be used in theconstruction of the first and second laminates.

A closer inspection of FIG. 10 reveals that the PWB 10″″ includes anumber of plated holes. The PWB 10″″ includes chimneys 128 that areholes filled with thermally conductive material. The chimneys are usedto transport heat from the surface of the PWB to the electrically andthermally conductive laminates within the PWB. The chimneys do notextend all the way through the PWB. If the chimneys contacted both thefirst and second laminates, then the chimneys could short circuit thePWB. The PWB 10″″ also includes through holes 130 lined withelectrically conductive material that are used to establish electricalconnections between the functional layers in the PWB. Where connectionsbetween the plated through holes and the first or second laminates arenot desired, then an annulus of dielectric material 132 such as an epoxyresin with a dielectric constant less than 6.0 at 1 MHz can be used toensure that an electrical connection does not exist between the laminateand the electrically conductive lining of the through hole.

A process in accordance with the present invention for manufacturing thePWB 10″″ illustrated in FIG. 10 is shown in FIG. 11A. The process 150commences with the step 152, which involves constructing two laminatesin accordance with the present invention are formed using the processdescribed above in relation to FIG. 2A. Power or ground regions are thenpatterned on the laminates in the step 154. The patterning electricallyisolates regions within the laminate, which can enable laminate tofunction as a ground or power plane within a PWB.

Once the patterning is complete, the laminates are subjected to oxidetreatment in the step 156. After oxide treatment, clearance holedrilling is performed in the step 158. Clearance hole drilling involvesdrilling holes in the laminate of a first diameter and filling theresulting holes with a dielectric material such as any of the resinsdescribed above with a dielectric constant less than 6.0 at 1 MHz. Priorto filling the drilled holes, they are inspected and cleaned using highpressure dry air.

Once the clearance holes have been drilled, the second lamination cycleis performed in the step 160. The second lamination cycle is similar tothe second lamination cycle described above in relation to FIG. 2A.After the second lamination cycle, chimney holes are drilled into thePWB in the step 162. Once the chimney holes have been drilled, thelinings of the chimney holes are lined with a thermally conductivematerial in the step 164. Preferably, the thermally conductive materialis copper. In other embodiments, any material with a thermalconductivity greater than 5 W/m.K can be used.

After the chimney holes have been lined, circuits are etched onto thelayers of metal that will be located within the interior of the finishedPWB are patterned in the step 166 and then subjected to oxide treatmentin the step 168.

Following the oxide treatment, the third lamination is performed in thestep 170. The third lamination involves aligning the two structuresproduced in the second lamination with additional prepreg layers tocorrespond with the layers of the PWB 10″″ illustrated in FIG. 10. Thelayers are then exposed to temperatures and pressures similar to thoseexperienced during the second lamination cycle.

After the third lamination cycle, the final through hole drilling isperformed in step 172. The final through hole drilling involves drillingholes through the entire PWB that have a second diameter, which is lessthan the first diameter described above. The through holes are thenlined in the step 174. Preferably, the through holes are lined withcopper. In other embodiments, the through holes can be plated withmaterials similar to those that can be used in the construction of thelayers of metal. If a through hole passes through one of the filledclearance holes in a laminate, then the lining of the through holes areelectrically isolated from the laminate in which the clearance hole isdrilled. If a through hole does not pass through one of the filledclearance holes in a laminate, then the lining of the through hole is inelectrical contact with the laminate.

An embodiment of a process for selecting the locations in which to drillchimney holes in a PWB is illustrated in FIG. 11B. The process 190includes a first step 192, which involves creating a model of thestructure of the PWB. The second step 194 involves adding a thermallyconductive material such as copper to the outermost layers of metal onthe model. The thermally conductive material is added to the model suchthat the thermally conductive material does not create any electricalcontacts with the circuits patterned onto the layers of metal on whichthe thermally conductive material is added.

Once the thermally conductive material has been added, the locations ofthe chimney holes are determined in the step 196. The locations of thechimney holes are determined by choosing a location on the surface ofthe PWB that lies within an area where thermally conductive material wasadded during step 194. The location is a suitable location for a chimneyif a hole of a specified diameter corresponding to the diameter of thechimney can be drilled through the PWB without intersecting any of theelectrical circuits patterned onto layers of metal internal to the PWB.Otherwise, the chosen location is unsuitable as a location for drillinga chimney hole. The number of locations that must be found is dependentupon the amount of heat required to be conducted away from the surfaceof the board.

An embodiment of a process for selecting the location of the filledclearance holes in the laminates is illustrated in FIG. 11C. The process200 includes a first step 202, which involves constructing a model ofthe PWB. The locations of the through holes in the PWB are used todetermine the locations in which the through holes intersect the groundplane laminate or the power plane laminate in the step 204. Once theselocations have been determined, the locations of the clearance holes arechosen in the step 206 as the locations where the through holesintersect the ground or power plane laminates and where an electricalconnection between the lining of the plated through hole and the groundor power plane laminate is undesirable.

A PWB in accordance with the present invention incorporating laminatesin accordance with the present invention and an additional carboncontaining layer is illustrated in FIG. 12. The PWB 10′″″ has a firstlaminate 120′, a second laminate 122′, an additional carbon containinglayer 210, prepreg layers 124′ and layers of metal 126′. Preferably, thefirst laminate forms a ground plane and the second laminate forms apower plane. The additional carbon containing layer 210, does not act asa ground or power plane and is electrically isolated from the laminatesand the layers of metal. The additional carbon containing layerincreases the thermal conductivity and stiffness of the PWB and improvesthe CTE of the PWB. Similar materials to those used in the constructionof the laminates, prepreg layers and layers of metal of the PWB 10illustrated in FIG. 1 can also be used to construct the laminates,prepreg layers and layers of metal of the PWB 10′″″ illustrated in FIG.12. The additional carbon containing layer can be constructed using thesimilar materials to those that can be used in the construction of thecarbon containing layers of the laminates illustrated at 12 in FIG. 1,12′ in FIG. 3, 12″ in FIG. 4, 12′″ in FIG. 5 and 12″″ in FIG. 6.

The PWB 10′″″ in FIG. 12 can be constructed using a processes similar tothose described above in relation to FIGS. 11A-11C. The only differenceis in the arrangement of the materials used in the construction of thethird lamination cycle and the fact that filled clearance holes mustalso be drilled in the additional carbon containing layer 210 so thatthe additional carbon containing layer is electrically isolated from thelinings of any through holes present in the PWB.

Although the embodiments described above have included a single or twolaminates in accordance with the present invention, one skilled in theart would appreciate that a PWB can be constructed including three ormore laminates using the processes described above.

1. A printed wiring board including multiple functional layers,comprising: a layer containing carbon; wherein at least the layercontaining carbon forms a functional layer of the printed wiring board.2. The printed wiring board of claim 1, wherein at least the layercontaining carbon forms a power plane.
 3. The printed wiring board ofclaim 1, wherein at least the layer containing carbon forms a groundplane.
 4. The printed wiring board of claim 1, wherein at least thelayer containing carbon forms a split plane.
 5. The printed wiring boardof claim 1, wherein the layer containing carbon includes woven carbonfibers.
 6. The printed wiring board of claim 1, wherein the layercontaining carbon includes at least one sheet of unidirectional carbonfibers.
 7. The printed wiring board of claim 1, wherein the layercontaining carbon has a dielectric constant greater than 6.0 at 1 MHz.8. The printed wiring board of claim 1, wherein the layer containingcarbon includes a carbon substrate impregnated with a resin.
 9. Theprinted wiring board of claim 8, wherein the resin is a thermallyconductive resin.
 10. The printed wiring board of claim 8, wherein theresin is an electrically and thermally conductive resin.
 11. The printedwiring board of claim 1, further comprising holes lined withelectrically conductive material extending through the printed wiringboard.
 12. The printed wiring board of claim 1, wherein at least one ofthe holes lined with electrically conductive material creates anelectrical connection between the functional layer formed by at leastthe layer containing carbon and another functional layer within theprinted wiring board.
 13. The printed wiring board of claim 1, furthercomprising: a first prepreg layer positioned above the substrate; asecond prepreg layer positioned below the substrate; a first layer ofelectrically conductive material positioned above the first prepreglayer; and a second layer of electrically conductive material positionedbelow the second prepreg layer.
 14. The printed wiring board of claim13, wherein the laminate formed by the layer containing carbon, thefirst and second prepregs and the first and second layers ofelectrically conductive material has a dielectric constant greater than6.0 at 1 MHz.
 15. The printed wiring board of claim 1, furthercomprising a second layer containing carbon.
 16. The printed wiringboard of claim 15, wherein at least the second layer containing carbonacts as a second functional layer of the printed wiring board.
 17. Theprinted wiring board of claim 15, wherein the second layer containingcarbon does not form part of a functional layer of the printed wiringboard.
 18. A method of constructing a printed wiring board, comprising:forming a stack of layers of electrically conductive material and atleast one layer containing carbon, where the layers of electricallyconductive material and layers of carbon are separated by layers ofdielectric material; forming circuits on the layers of electricallyconductive material; drilling holes through the stack and lining theholes with electrically conductive material, where the holes are drilledin locations that enable the layer containing carbon to form afunctional layer within the printed wiring board.
 19. The method ofclaim 18, wherein the holes are drilled in locations so that the layercontaining carbon forms a power plane in the printed wiring board. 20.The method of claim 18, wherein the holes are drilled in locations sothat the layer containing carbon forms a ground plane in the printedwiring board.
 21. The method of claim 18, wherein the holes are drilledin locations so that the layer containing carbon forms a split plane inthe printed wiring board.
 22. The method of claim 18, wherein theelectrically conductive layers and dielectric layers are formed fromprepregs and clad and unclad laminates.
 23. The method of claim 22,wherein the stack is formed by at least one lamination cycle.
 24. Themethod of claim 23, wherein at least some of the circuits on theelectrically conductive layers are formed on the laminates prior to thelamination cycles.
 25. The method of claim 24, wherein at least some ofthe circuits on the electrically conductive layers are formed on thelaminates after at least one of the lamination cycles.
 26. The method ofclaim 18, further comprising: forming regions of thermally conductivematerial on regions of an outer layer of the stack that do not containany part of a circuit; wherein at least some of the holes drilledthrough the stack are drilled through the regions of the thermallyconductive material formed on the outer layer of the stack.